Composition for igniter for rectifiers



Patented Apr. 6, 1948 COMPOSITION FOR IGNITER FOR RECTIFIERS Guy H.Fetterley, Chippawa, Ontario, Canada, as-

signor to Norton Company, Worcester, Mass., a corporation ofMassachusetts No Drawing. Application January 17, 1946, Serial No.641,872

7 Claims. 1 The invention relates to a composition of matter and withregard to its more specific features to a composition for igniter tipsfor rectifiers.

One object of the invention is to provide an inexpensive igniter tip.Another object of the invention is to provide a general composition forigniter tips from which articles with predictable properties can bereadily made. Another object of the invention is to provide acomposition of the type indicated which has the desired resistivity.Another object is to provide igniter tips of long life.

Another object is to provide an igniter with small negative coefficientof resistance. Another object is to provide a refractory igniter withthe desired resistivity and a small negative coefficient of resistance.Another object of the invention is to provide a refractory compositionof matter that is not wetted by mercury, that has a resistivity in therange from .5 to 5. ohm-inches, and a negative coefficient of resistancenot greater than three-quarters of one percent per degree Centigrade.Another object is to provide a composition for igniter tips of desiredresistivity which resistivity shall not drop excessively on the surfaceof the igniter tip during use thereof. Another object of the inventionis to find a satisfactory bond for titanium nitride to make refractorysemi-conductors of readily precalculated resistance. Another object isto provide a strong refractory material which can be readily molded.Another object is to provide a material useful for the manufacture ofresistors, for example heating elements for industrial furnaces. Anotherobject is to make a refractory and corrosion resistant material. Otherobjects will be in part obvious or in part pointed out thereinafter.

The invention accordingly consists in the com binations of elements andin the several steps and relation and order of each of said steps to oneor more of the others thereof, all as will be illustratively describedherein, and the scope of the application of which will be indicated inthe following claims.

For a description of a mercury arc rectifier known as the ignitron animportant element of which is an igniter tip, reference may be had to anarticle by Joseph Slepian entitled, The ignitron, a new mercury arcpower converting device in Transactions of the Electrochemical Society,volume 69, pp. 399-415 (1936). The ignitron is a mercury arc rectifier.In any mercury arc rectifier the electrons are emitted from what isknown as the cathode spot. This cathode spot is useful only during thecurrent carrying period. Its existence at other times promotes arc-backsor back-fire. In the ignitron a semiconducting rod called the igniter isimmersed part way into the mercury pool and is connected to conveycurrent from a shunt circuit into the pool during the current carryingperiod. Thus a properly timed current pulse is given to the igniter atthe beginning of each desired current carrying period. One convenientarrangement for thus imparting the proper current pulses to the igniterinvolves connecting the igniter terminal to the anode of the rectifierexternally through a small thermionic cathode tube, which is a simplerectifying tube. The cathode spot is thus maintained during the currentcarrying period and at the end of each such period the cathode spotautomatically is extinguished. Such construction makes a highlyefiicient rectifier and indeed has permitted rectification in circuitsand for uses not hitherto possible or practicable. The life of anignitron is also usually greater than that of previous types ofrectifiers. Many thousands of ignitrons are now in successful use andhave been for years.

The efficiency and continued operation of an ignitron, however, aredependent upon the use of an igniter rod of proper characteristics. Thedesirable characteristics of igniter rods are that they shall not be wetby mercury, they shall have a resistivity of the order of 5. to .5ohm-inches, and that the resistivity shall not break down on thesurface. Especially desirable features are indicated in the objectshereinbefore given.

Igniter tips, sometimes referred to as igniter rods or merely asigniters, may be made in various shapes or dimensions but as anillustrative example an igniter tip may consist of a cylinder long, A"in diameter, capped by an integral cone long, the cylinder merging intothe base of the cone. A convenient way to make such igniters is to makea cylinder somewhat over '7 long, and in diameter, grind a conical tipwith an altitude of then cut off the cylindrical part to the desiredlength, in this case inch.

I have found that superior results are achieved by making igniters outof titanium nitride (TiN) bonded with any suitable non -conducting bondand in this connection non-conducting means substantially dielectric.Many different bonds might be used such as various ceramics, oxides ormixtures of oxides and in this connection alumina (A1203) which can bemolded at around '1800 C. under pressure.

the balance beryl.

3 1800 C. is particularly useful. preferred bond is beryl.

Beryl is a natural mineral and I obtain good results using the naturalmaterial. Beryl is a variable mineral of a general composition sometimesWIlti/fll 3BO.A12O3.6Si02. I can use all forms of beryl which arerecognized as being substantially pure beryl. Some emeralds arechemically pure beryl in crystalline form and indeed beryl as such isalways crystalline.

Beryl has a melting point in a range between 1400 and 1500 C. and moldsreadily over a range of temperatures below 1500 C. In this respect itacts like a glass. I prefer that whatever bond is used it should bemoldable between 1000 C. and Beryl is an electric insulator in thedielectric range Which is high relative to the resistivity of titaniumnitride. It is dense, mechanically strong and makes a good bond.

Titanium nitride is not wetted by mercury, neither as such nor when itis bonded with beryl. It fuses at around 2900 C. Its resistivity at 20C. is about .21 x ohm-centimeters. This is the same as .0827 x 10-ohm-inches. When titanium nitride is molded With 75% beryl, theresistivity is about 5. ohms to the inch. When titanium nitride ismolded with 65% beryl, the resistivity is about .5 ohm-inches. Since theabove is the desired range for igniters I prefer the limits 25% to 35%IiN, the parts being by weight. One feature of the invention is that byvarying the percentage, I can control the resistivity within fairlyclose limits as compared with other compositions.

For the manufacture f a rod from which to make an igniter, I may proceedas follows;

I provide a graphite mold of cylindrical shape having preferably anumber of bores all parallel to the axis of the cylinder and each one /1of an inch in diameter. I provide graphite mold plungers each A of aninch in diameter. The mold bores are filled with a mixture consisting offrom 25 to 35% by weight titanium nitride,

The particle size of the titanium nitride is preferably small, forexample around 10 microns. The beryl is also in powdered form. Forexample, I may crush it and select the particles that pass a screen 325meshes to the linear inch. I prefer that the beryl particles be 40microns or finer, the finer the better.

The two ingredients are well mixed in a ball mill and then charged intothe bores of the multiple graphite mold, one set of plungers having beeninserted before the mold is charged. Then the other set of plungers isinserted and the mold is placed within the graphite resistor tube of anelectrical furnace such as disclosed in the U. S. patent to R. R.Ridgway No. 2,125,588. Pressure is then applied against the moldplungers by means of the graphite plungers 535 of the Ridgway patentwith or without the intermedaite graphite spacers I73 of said patent,and coincidentally or previously the furnace is energized to heat thegraphite resistor tube 50 of the patent Which heats the mold and themixtures contained therein.

I desirably use a pressure of 2500 pounds to the square inch becausethese furnaces are customarily operated at such pressure andcalculations may be readily made thereat. However, other pressures mightbe used. I heat the material to a temperature of around 1400 to 1500 C.This is not indefinite because the plungers move slightly when thematerial sinters, and after the tem- However, my

4 perature has reached 1400 C. and the plungers have moved, the heat ismaintained for about two minutes and then the circuits are opened. Whenthe mold has cooled to around 500 C., the pressure may be released.

After the mold and the furnace have cooled sufficiently, the furnaceisopened and the mold is removed. The mold is then cut open and thecylindrical pieces of titanium nitride bonded with beryl are extracted.These may then be ground and cut to the desired size and shape asalready indicated. Igniter tips made as described are dense and strongmechanically. The resistivity wil1 be in the range stated, namely fromabout .5 to 5. ohm-inches.

One feature of the invention resides in the fact that titanium nitridehas a positive temperature coefficient of resistance. Most othersemi-conductors have a negative temperature coefficient of resistance.The combination of titanium nitride bonded With beryl has a negativecoeiiicient of resistance but it is small. Measured in percentage changeper degree centigrade this coefficient is around -0.25% to 0.40%. A lowtemperature COEfilClGllt of resis ance is desirable in an igniter tip.

The titanium nitride used to make the composition of this invention maybe made as described in the literature or procured on the open market.It is desirable to use fairly pure material, however high purity is notessential. variety I have used was about pure, the remainder beingmostly titanium oxide or oxides with probable traces of titaniumcarbide. This is satisfactory material for the purpose of makingigniters.

The peak current through an igniter is frequently of the order of 20amperes at around 50 volts. The pulse rate, of course, depends upon thefrequency of the circuit which may be 25 or 50 but more usually 60cycles. The current at the contacts between the igniter and the mercuryrises to very high density even to several thousand amperes per squarecentimeter and the voltage drop at the contact may be of the order of 10volts. These requirements rule out the use of ordinary metals forigniters and require an igniter to be refractory. Many materials whichhave hitherto been tried for igniters have a relatively short life. Thisis apparently because they are not resistant to heat shock. The ignitersthat are made in accordance with this invention last a long time andresult in efficient operation of the ignitrons.

In the ignitor tips as formed by molding, the titanium nitride isdispersed at random throughout a matrix of beryl. The titanium nitrideoccurs in massive yellow grains and golden dendrites and in aggregates.Thus, it appears that many of the crystals. which Were originally of theorder of 10 microns in size, have grown together to make large crystals.In most cases, the pores of the aggregates are filled with beryl, butthere are some voids.

While an important use for the composition of the invention is for themanufacture of igniter tips, the composition TiN+ beryl is a strongrefractory material in Wide proportions of TiN to beryl and may be usedto make other articles. Thus for resistors, Till may be bonded withberyl in proportions beyond the limits of 25% to 35% Till in eitherdirection. For example, resistor bars for high temperature electricfurnaces can be made out of the composition of this invention. Thedesired resistivity for such bars is of the order The of .5 to .02ohm-inch. To obtain a resistivity of .02 ohm-inch, 50% TiN, 50% beryl isapproximately the right proportion. Resistors of resistivity muchgreater than 5 ohm-inches may be made, eVen up to resistivities ofmegohm-inches, by increasing the amount of beryl above 75%.

It will thus be seen that there has been provided by this invention anarticle and a composition in which the various objects hereinabove setforth together with many thoroughly practical advantages aresuccessfully achieved.

As many possible embodiments may be made of the above invention and asmany changes might be made in the embodiment above set forth, it is tobe understood that all matter hereinbefore set forth is to beinterpreted as illustrative and not in a limiting sense.

I claim:

1. An igniter in which the semi-conductive element is titanium nitrideof a purity of about 85% and upwards and in an amount in the range fromabout 25% to 35% bonded with a refractory bond into a dense strongarticle with resistivity of the order of .5 to 5. ohms-inches at 20 C.,the titanium nitride being well dispersed throughout the bond.

2. An igniter consisting of titanium nitride bonded with beryl, thetitanium nitride being 6 about 85% pure and upwards and consisting offrom about 25% to about by weight of the article, the titanium nitridebeing in finely divided form and Well dispersed through the beryl bond.

3. As a new composition of matter an intimate sintered mixture oftitanium nitride and beryl.

l. As a new composition of matter, titanium nitride in granularcrystalline form bonded with beryl.

5. An igniter of titanium nitride bonded with beryl.

6. An igniter of titanium nitride bonded With beryl having a negativecoefficient of resistance no greater than 375% per degree centigrade.

'7. An igniter of titanium nitride bonded with beryl having aresistivity of from .5 to 5. ohminches.

GUY H. FETTERLEY.

REFERENCES CITED FOREIGN PATENTS Country 4 Date Great Britain Oct. 12,1909 Number

